Medical fixation devices with improved torsional drive head

ABSTRACT

The present invention provides a fixation device that includes an elongate shank defining a longitudinal axis and having at least one engaging member for applying the fixation device within tissue and securing the fixation device in the tissue once implanted formed thereon, and a drive head having a proximal end, a distal end and a radial cross-sectional geometry, where the drive head is mated to the elongate shank, and includes at least one anti-rotational member integral therewith, Fixation device kits utilizing the fixation device, and methods of fixation in tissue are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/170,377 filed on Jun. 29, 2005 and entitled “Medical Fixation Deviceswith Improved Torsional Drive Head,” which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to medical fixation devices havingimproved physical properties, more particularly, to biologicallycompatible fixation devices requiring torsional forces to secure theirapplication within body tissue.

BACKGROUND

Within the medical field, fixation devices are used in different ways toassist in the reconstruction of damaged body tissue. A fixation devicemay be used to directly secure tissue in close approximation toneighboring tissue to effect healing such as is the case with meniscalfixation devices, or fracture fixation pins, screws, or wires. Othertypes of fixation devices are intended to provide mechanical stabilityand load sharing during the healing process, as when a graft is securedin a bone tunnel for ACL reconstruction. Fixation devices may also beused in conjunction with other device hardware such as plates, rods, orvarious other connecting members known in the art as part of an implantassembly, such as with a spinal screw fixing a plate to the vertebralbody, spinal pedicle screws connected to posterior rod assemblies toname just a couple examples. Other fixation devices are used to anchorsuture to bone so the suture can be used to secure injured soft tissues.

Fixation devices typically have an elongate body, and one or moreengaging feature(s) for retaining the device within body tissue or aspart of a device assembly. The fixation device may either be insertedinto body tissue directly, through a preformed hole with or without theaid of a tap, or as part of a device assembly within the bone cavity,such as with a screw/sheath assembly. Oftentimes fixation devicesrequire the application of torsional forces from an insertion tool atone end of the implant to secure their application into body tissue, aswith screw-type implants. Insertion tools are typically formed from anelongate shank having a mating feature formed on a distal end thereoffor mating with a corresponding mating element formed on or in the headof the fixation device. One common type of driver tool includes ahexagonal-shaped or square-shaped socket for receiving a correspondinghexagonal-shaped or square-shaped head of a fixation device.

Certain conventional fixation devices and their drivers have somedrawbacks. Device heads with hexagonal or square shaped cross-sections,for example, tend to have a relatively low stripping strength, meaningthat under relatively small torque loads the drive head may bepermanently damaged and torque transfer thus inhibited. If the headshape decreases the amount of material on the fixation device head oranchor head that interfaces with the driver, then the amount of materialthat needs to yield or be “stripped” from the drive head is reduced,thus reducing the stripping strength of the head.

Conventional fixation device heads also tend to have a relatively lowfailure torque, which can result in shearing of the head duringinsertion or stripping of the head elements necessary to transfer torqueto the device. This type of failure can also be caused by the geometryof the head, which can reduce the overall cross-sectional area of thedrive head. Fixation devices were historically constructed ofimplantable metals and alloys which afforded sufficiently high tensileand torsional strength to withstand the rigors of insertion, but theimplant remained in the body for prolonged periods of time. Polymer,ceramic, or composite material systems, both biodegradable andnon-biodegradable, have been developed for similar applications, buttypically have lower tensile and torsional strength than metalcounterparts, thus increasing the risk of device failure duringapplication of high tortional torque loads during implantation in thebody, as described above. More recently, biodegradable compositematerial systems have been developed that incorporate filler materialswithin the polymer matrix, such as calcium phosphate particles, whichare osteoconductive. These filled systems may have further reducedtensile or torsional properties compared to unfilled polymer systems.Thus there is a need for an improved drive head for implantable fixationdevices that has higher torsional resistance to strippage or shearingoff.

One option to increase the failure torque of a fixation device is toincrease the size of the head. Large device heads, however, require alarge driver tool, which in turn requires a relatively large bone tunnelto be formed in the bone. This is particularly undesirable where thebone tunnel is to be formed in the cancellous bone, and where theprocedure is minimally invasive and must traverse through a cannula orarthroscope. Accordingly, most fixation devices are adapted for use witha relatively small driver tool, and thus they have a relatively smalldrive head, which can result in a low failure torque and a low strippingstrength, particularly in harder bone applications. A drive head ofimproved torsional strength is desirable to reduce the risk ofdeformation during insertion. Additionally, a drive head more resistantto deformation upon application of torsion may make a revision procedureeasier, as there are some instances where torque driven devices need tobe backed out and perhaps even reinserted.

Accordingly, there remains a need for fixation devices having improvedphysical properties, and in particular having a high failure torque anda high stripping strength.

SUMMARY

The present invention provides a fixation device including an elongateshank that includes proximal and distal ends and defines a longitudinalaxis. The shank further includes formed thereon at least one engagingmember for facilitating placement of the device in body tissue, andsecuring the device in the tissue once implanted. The fixation devicealso includes a drive head having a proximal end, a distal end, and aradial cross-sectional geometry; where the distal end is mated to theproximal end of the elongate shank. The drive head includes at least oneanti-rotational member having a longitudinal cross-sectional geometryintegral with the drive head. The invention is also directed to fixationdevice installation kits containing the fixation device and a drivetool, as well as methods for attachment of tissue.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1A is a perspective view of a fixation device of the presentinvention.

FIG. 1B is another perspective view of the fixation device shown in FIG.1A.

FIG. 1C is an enlarged, perspective view of the drive head portion ofthe fixation device shown in FIG. 1A.

FIG. 1D is a top view of the fixation device shown in FIG. 1A.

FIG. 2 is a perspective view of an alternate embodiment of the drivehead portion of the present invention.

FIG. 3 is a perspective view of alternate embodiment of the drive headportion of the present invention.

FIG. 4 is a perspective view of an alternate embodiment of the drivehead portion of the present invention.

FIG. 5 is a perspective view of an alternate embodiment of the drivehead portion of the present invention.

FIG. 6A is a side view of one embodiment of a driver tool in accordancewith the present invention.

FIG. 6B is an end view of the distal-most end of the driver tool shownin FIG. 6A.

FIG. 7A is a perspective view of one embodiment of the fixation deviceand driver tool where the head of the device is not mated with thesocket of the driver tool.

FIG. 7B is a perspective view of one embodiment of the fixation deviceand driver tool where the head of the device is mated with the socket ofthe driver tool.

DETAILED DESCRIPTION

The present invention provides a fixation device including an elongateshank defining a longitudinal axis and having at least one engagingmember formed therewith to engage body tissue and facilitate placementof the device within the tissue and to secure the device in the tissueonce implanted. The fixation device also includes a drive head forapplying torsion to the elongate shank having a proximal end and adistal end and which is mated to the elongate shank so as to transferthe torsion to the elongate shank, thereby providing for placement ofthe device in tissue. The drive head may have a circular or asubstantially non-circular radial cross-sectional geometry, for exampleoval, and includes at least one anti-rotational member (ARM) formedintegrally therewith to provide for increased transfer of the torsion tothe shank. In one embodiment the fixation device comprises a drive headof circular or substantially non-circular radial cross-sectionalgeometry with at least one ARM integral therewith. In a secondembodiment, the fixation device comprises a drive head of circular orsubstantially non-circular radial cross-sectional geometry with multipleARMs located on either side of a plane of symmetry for ease of inserterapplication. The ARMs are of configuration and dimension effective toprovide a mating fit with the driver tool in order to ensure efficienttransfer of torsion from the drive head to the shank. The presence ofthe ARMs provides high failure torque and high stripping strength.

In other aspects, a fixation device and installation kit is provided,including at least one fixation device and a cylindrical driver tool forcooperating with the fixation device. The fixation device has a shankwith engaging member formed thereon and defining a longitudinal axis. Adrive head is formed on the shank and has a circular or substantiallynon-circular radial cross-sectional geometry, such as oval, and at leastone ARM formed integral therewith. The cylindrical driver tool has adistal end with a socket formed therein having a shape adapted toreceive and engage the drive head of the fixation device. The ARM(s)also provide positive mating with the driver tool, such as a key inkeyway configuration, to reduce driver slip-off, especially duringoff-angle insertions. In an exemplary embodiment, the driver tool has anouter diameter that is equal to or less than an outer-most diameter ofthe fixation device.

As shown in FIGS. 1A-1D, where like numbers refer to like features, thepresent invention generally provides a fixation device 10, includingelongate shank 12 defining longitudinal axis A and having at least oneengaging member 20 formed thereon. In the embodiment shown, engagingfeature 20 is a helical thread. Drive head 30 has proximal end 32 anddistal end 34 mated to elongate shank 12 at proximal end 14. In thiscase, drive head 30 has a substantially rectangular radialcross-sectional geometry, though drive head 30 could have substantiallycircular, rectangular, square, hexagonal, flattened oval or oval radialcross-sectional geometries, where the radial cross-section is defined asthe cross-section perpendicular to longitudinal axis A.

The configuration of drive head 30 includes at least one ARM 36protruding from and integral with drive head 30 and extending fromdistal end 34 to proximal end 32 of drive head 30. The configuration ofdrive head 30 with ARM 36 is particularly advantageous in that itprovides fixation device 10 with improved physical properties, includinghigh failure torque and high stripping strength.

Elongate shank 12 of fixation device 10 can have a variety ofconfigurations and can include a variety of different engaging members20 formed thereon. FIGS. 1A and 1B illustrate an exemplary embodiment offixation device 10 having shank 12 including core 18 with single helicalthread 20 extending around core 18 from proximal end 14 to distal end 16of shank 12. Thread 20 includes proximal and distal facing flanks 22, 24that extend between base 26 and substantially flattened crest 28. Thread20 defines a major diameter d₂ of shank 12, which can vary along thelength of shank 12, although major diameter d₂ is substantially constantalong a substantial portion of shank 12. Threads 20, however, can taperat the distal portion of shank 12 to terminate at apex 29 of shank 12.Core 18 of shank 12 defines a minor diameter d₁ that can also besubstantially constant or can vary along the length of shank 12. Asshown in FIG. 1A, core 18 tapers from proximal end 14 to distal end 16.Once having the benefit of this disclosure, a person skilled in the artwill appreciate that shank 12 shown in FIG. 1A is merely an exemplaryembodiment of a shank 12, and that a variety of different shanks havingdifferent tissue-engaging members can be used with fixation device 10 inaccordance with the present invention.

Drive head 30 of fixation device 10 is shown in more detail in FIGS. 1Cand 1D, and is attached to, or formed integrally with, shank 12. Therelatively small size of widths W₁ and W₂ of drive head 30, as comparedto major diameter d₂ of shank 12, is particularly desirable so thatdrive head 30 will not require a larger cavity to be formed in thetissue than necessary. Drive head 30 further includes length L_(h)(shown in FIG. 1A) that extends between proximal and distal ends 32 and34 thereof. Length L_(h) of drive head 30 can vary, although lengthL_(h) of drive head 30 may be optimized to allow the drive head to bereceived within a driver tool and to be driven into bone withoutshearing off. Drive head 30 has ARM 36 extending along length L_(h)between distal end 34 and proximal end 32 of drive head 30.

FIG. 2 shows an alternate embodiment of drive head 40 of a fixationdevice according to the present invention. In this embodiment, theradial cross-sectional geometry of drive head 40 is substantially squarein shape with ARM 46 thereon that originates at distal end 44 andextends to proximal end 42. As shown, ARM 46 is tapered from proximalend 42 toward distal end 44. Once having the benefit of this disclosure,those skilled in the art will recognize that ARM 46 may be of otherlongitudinal cross-sectional geometries, e.g. parabolic, wedge, etc.without deviating from the scope of the invention, and that the ARM mayextend only partially from distal end 44 toward proximal end 42 (notshown). Additionally, while not shown, ARM 46 may taper in the oppositeorientation, i.e. from distal end 44 to proximal end 42.

FIG. 3 shows a further embodiment where drive head 50 contains four ARMs56 a, 56 b, 56 c and 56 d positioned on opposing faces of drive head 50.A plurality of ARMs spaced equidistant around the drive head, such asdepicted here, may be desirable from a procedural standpoint where thehead possesses a plane of symmetry. In the embodiment depicted in FIG.3, the radial cross-sectional geometry of drive head 50 is square, andthus has two planes of symmetry. With ARMs on either side of a plane ofsymmetry, rotational alignment of a mating inserter with respect to theimplant is further alleviated. Multiple ARMs may also afford furtherimproved physical properties. Once having the benefit of thisdisclosure, those skilled in the art will soon recognize other possibleconfigurations with multiple ARMs on either side of a plane of symmetrykeeping within the scope of the invention.

In FIG. 4, an alternate embodiment of drive head 70 is shown. Drive head70 is substantially oval in radial cross-sectional shape defining aminor diameter X₁ and a major diameter X₂. Generally oval is known toinclude flattened ovals and ovals with flat portions perpendicular tothe minor X₁ or major X₂ diameters of drive head 70. In an exemplaryembodiment, minor diameter X₁ of head 70 is about three-fourths the sizeof major diameter X₂ of head 70, and major diameter X₂ of head 70 isequal to or less than minor diameter d₁ of shank 62. Drive head 70contains ARM 76 integral therewith. ARM 76 originates at distal end 74of driver head 70 and extends towards proximal end 72 for a total lengthof L_(r). The length L_(r) is less than or equal to the length L_(h).Once having the benefit of this disclosure, those skilled in the artwill soon recognize other possible configurations where the ARM mayoriginate at proximal end 72 and extend towards distal end 74 for alength of L_(r) (not shown).

FIG. 5 shows another alternative embodiment of drive head 80. Drive head80 is circular in radial cross-section and contains ARM 86 integraltherewith. ARM 86 extends from distal end 84 towards proximal end 82.

For placement of fixation devices of the present invention into tissue,fixation devices can be driven into tissue using a driver tool, such asshown in FIGS. 6A-6B. Driver tool 90 can have a variety of shapes andsizes, but typically includes elongate shaft 92, having proximal handleportion 94 and distal end 98 having socket 96 formed therein and adaptedto seat in mating relationship with the drive head of fixation devicesof the present invention. As shown in FIGS. 7A-7B, socket 96 of drivertool 90 has an overall square shape and includes opposed ARM-engagingelement 95 to engage and cooperate with ARM 116 once drive head 110 offixation device 100 is placed in cooperation with socket 96 of drivetool 90. The shape of socket 96 and ARM-engaging elements 95 form aclose fit with drive head 110 and cooperate with ARM 116 in such a wayas to provide the mated relationship of drive head 110 within socket 96.The size and configuration of the socket 96 in relationship to drivehead 110 and ARM 116 should be sufficient to provide a secure fitbetween drive head 110 and driver tool 90, and to prevent rotation ofdriver tool 90 with respect to fixation device 100. Driver tool 90 canalso contain an inner lumen (not shown) extending there through forreceiving free ends of suture.

Suitable materials from which fixation devices may be formed includebiocompatible polymers selected from the group consisting of aliphaticpolyesters, polyorthoesters, polyanhydrides, polycarbonates,polyurethanes, polyamides and polyalkylene oxides. The present inventionalso can be formed from biocompatible metals, glasses or ceramics, orfrom autograft, allograft, or xenograft bone tissues. Fixation devicescan be further comprised of combinations of metals, ceramics, glassesand polymers.

The biocompatible materials can be biodegradable or non-biodegradable.Biodegradable materials, such as polymers, readily break down into smallsegments when exposed to moist body tissue. The segments then either areabsorbed by the body, or passed by the body. More particularly, thebiodegraded segments do not elicit permanent chronic foreign bodyreaction, because they are absorbed by the body or passed from the body,such that the body retains no permanent trace or residue of the segment.

In one embodiment, the device comprises biodegradable aliphatic polymerand copolymer polyesters and blends thereof. The aliphatic polyestersare typically synthesized in a ring opening polymerization. Suitablemonomers include but are not limited to lactic acid, lactide (includingL-, D-, meso and D,L mixtures), glycolic acid, glycolide,epsilon-caprolactone, p-dioxanone (1,4-dioxan-2-one), and trimethylenecarbonate (1,3-dioxan-2-one).

In another embodiment, the materials comprising the devices will bebiodegradable glasses or ceramics comprising mono-, di-, tri-,alpha-tri-, beta-tri-, and tetra-calcium phosphate, hydroxyapatite,calcium sulfates, calcium oxides, calcium carbonates, magnesium calciumphosphates, phospate glasses, bioglasses, and mixtures thereof.

In another embodiment, the materials comprising the devices can becombinations of biodegradable ceramics and polymers. Composites areprepared by incorporating biodegradable ceramic reinforcements such asfibers, short-fibers, or particles in a biodegradable polymer matrix.

Some particularly useful composites are 30 weight percentbeta-tricalcium phosphate particles in 70 weight percent poly(lacticacid), or 30/70 beta-TCP/PLA, and 30 weight percent beta-tricalciumphosphate particles in 70 weight percent poly(lactide)/poly(glycolide)copolymer (mole ratio lactide to glycolyde 85/15), or 30/70beta-TCP/(85/15 PLGA).

In another embodiment of the present invention, the polymers and blendscan be used as a therapeutic agent release matrix. To form this matrix,the polymer would be mixed with a therapeutic agent prior to forming thedevice. The variety of different therapeutic agents that can be used inconjunction with the polymers of the present invention is vast.Therapeutic agents which may be administered via the pharmaceuticalcompositions of the invention include growth factors, including bonemorphogenic proteins (i.e. BMP's 1-7), bone morphogenic-like proteins(i.e. GFD-5, GFD-7 and GFD-8), epidermal growth factor (EGF), fibroblastgrowth factor (i.e. FGF 1-9), platelet derived growth factor (PDGF),insulin like growth factor (IGF-I and IGF-II), transforming growthfactors (i.e. TGF-beta I-III), vascular endothelial growth factor(VEGF); and other naturally derived or genetically engineered proteins,polysaccharides, glycoproteins, or lipoproteins.

Matrix materials for the present invention may be formulated by mixingone or more therapeutic agents with the polymer. Alternatively, atherapeutic agent could be coated on to the polymer, possibly with apharmaceutically acceptable carrier. Any pharmaceutical carrier can beused that does not dissolve the polymer. The therapeutic agent may bepresent as a liquid, a finely divided solid, or any other appropriatephysical form. Typically, but optionally, the matrix will include one ormore additives, such as diluents, carriers, excipients, stabilizers orthe like.

Methods for using a fixation device in accordance with the presentinvention are also provided. The medical fixation device is attached toa driver tool, the neighboring tissue segments are approximated, themedical fixation device is inserted into the approximated neighboringtissue segments, and the driver tool is removed.

The fixation devices disclosed herein may be used in different ways toassist in the reconstruction of damaged body tissue. They may be used todirectly secure tissue in close approximation to neighboring tissue toeffect healing. They may provide mechanical stability and load sharingduring the healing process, as when a graft is secured in a bone tunnelfor ACL reconstruction. Fixation devices of the present invention mayalso be used in conjunction with other device hardware such as plates,rods, or various other connecting members known in the art as part of animplant assembly.

The invention claimed is:
 1. A surgical method, comprising: positioningan anti-rotational member that is monolithic with a solid drive head ofa medical fixation device within an opening extending through a sidewallof a driver tool to attach the driver tool to the medical fixationdevice, the drive head being a rectangular block, and theanti-rotational member being a rectangular block having a volume smallerthan a volume of the drive head's rectangular block, the anti-rotationalmember extending proximally from a distal-most end of the solid drivehead and extending laterally from the solid drive head such that thedrive head is positioned within a distal end of the driver tool when theanti-rotational member is positioned in the opening in the sidewall ofthe driver tool; inserting a shank extending distally from the drivehead of the medical fixation device through at least one soft tissuesegment, the shank having a thread that terminates distal of the soliddrive head; and implanting the shank in bone to anchor the at least onesoft tissue segment in the bone.
 2. The method of claim 1, wherein anelongate shaft of the driver tool has a maximum outer diameter notgreater than a major diameter of the shank.
 3. The method of claim 1,wherein the soft tissue segment is a graft.
 4. The method of claim 1,wherein implanting the shank in bone to anchor the soft tissue segmentin the bone is for anterior cruciate ligament (ACL) reconstruction. 5.The method of claim 1, wherein the anti-rotational member extendsproximally from the distal-most end of the solid drive head to aproximal-most end of the drive head.
 6. The method of claim 1, whereinthe anti-rotational member extends proximally from the distal-most endof the solid drive head to a location that is distal to a proximal-mostend of the drive head.
 7. The method of claim 1, wherein a proximal-mostend of the shank is attached to the distal-most end of the solid drivehead.
 8. The method of claim 1, wherein the anti-rotational member issolid.
 9. A surgical method, comprising: inserting a drive head of amedical fixation device into a distal end of a driver tool, the drivehead is defined by a first rectangular block having a second rectangularblock extending laterally outward therefrom, wherein the insertion ofthe drive head into the distal end of the driver tool causes the firstrectangular block to be fully enclosed within the distal end of thedriver tool and the second rectangular block to extend through anopening formed in a sidewall of the driver tool; inserting a shankextending distally from the drive head of the medical fixation devicethrough at least one soft tissue segment, the shank having a thread thatterminates distal of the solid drive head; and implanting the shank inbone to anchor the at least one soft tissue segment in the bone.
 10. Themethod of claim 9, wherein the soft tissue segment is a graft.
 11. Themethod of claim 9, wherein implanting the shank in bone to anchor thesoft tissue segment in the bone is for anterior cruciate ligament (ACL)reconstruction.
 12. The method of claim 9, wherein the secondrectangular block extends along an entire longitudinal length of thefirst rectangular block, and the second rectangular block has a volumethat is less than a volume of the first rectangular block.
 13. Themethod of claim 9, wherein the first and second rectangular blocks areeach solid.